Brief summary of Ben Franklins electrical experiments



Tuesday Jan. 18, 2011

Today's class was a brief overview of some of the important historical developments in Atmospheric Electricity (mostly contributions from Benjamin Franklin)

Lightning was feared and often seen as a form of punishment (though also sometimes associated with fertility - eg. birth of twins)

“symbols based on the concept of fire are among the oldest pictorial representations of lightning.”  ref (1, see the end of today's notes for a complete reference)

 

“In many ethnological representations from prehistoric times lightning is depicted …. as a stone falling from heaven or a stone axe hurled from the skies.”  ref(1) 

“French peasants carry a “pierre de tonnerre” in their pockets to ward off lightning during thunderstorms” ref (1)

Next time I'm in France I'm going to try to find one of those, though I'm beginning to wonder if this isn't the same as a fulgurite.

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We spent most of the period describing some of the contributions made by Benjamin Franklin (born Jan. 17, 1706 in Boston, died April 17, 1790 in Philadelphia).  I know very little about him but have sat in on several lectures delivered by Dr. Krider.  A large part of what is in today's notes comes from notes that I took during his lectures.  You'll also find an article about Franklin written by Dr. Krider here. 

Franklin actively experimented with electricity for only a few years starting in the 1740s & continuing into the early 1750s.

There were many people in the American Colonies and Europe that were interested in and actively studying electricity at the time.  Franklin (emulated Newton) & performed experiments to test his theories.

Electrostatic machines (friction machines) were fairly common at the time (see Wikipedia) 

Peter Collinson – unpaid London agent of the Philadelphia Library Company gave Franklin a glass-rubbing tube in late 1746. 

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Early machines often used some material (silk or wool?) rubbing against a spinning glass sphere or a glass cylinder.  The glass would acquire one charge, the rubbing surface the other polarity.  (here's my interpration of the French phrase at the bottom of the figure: "I know, where is best found this almost magical virtue, wisely named electric; young beauties it's in your eyes.")

Construction of an electrostatic machine of some kind would make a suitable project for this class. 

“By being removed so far from the European centres of experimentation and discussion of electrical events.  Franklin was able to view his own observation with a freshness not encumbered by the earlier notions of others.  He therefore regarded an electrically undisturbed body as being under neutral charge or as in a state of electrical equilibrium." ref (2)

Franklin believed (correctly) that rubbing two materials together did not create electricity.  Rather, the rubbing somehow or another "grabbed onto" and separated charges that already were part of the neutral materials.  Material 1 might "tear" electrons from material 2.  Material 1 would become negatively charged and material 2 would be left positively charged.

Priestley explains this more clearly and succintly:

“Dr. Franklin had discovered … that the electric matter was not created but collected by friction, from the neighbouring non electric bodies”

Priestley was the first historian of electrical science.  His “History and Present State of Electricity” appeared in London in 1767.

Leyden Jar capacitors were used to store electrical charges produced by the friction machines.  A "dissectable" Leyden jar was shown in class. 

 

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The Wimhurst generator was developed between 1800 and 1883 by a British inventor.  Leyden jars are shown on the right and left (photo from Wikipedia)

We used a Van de Graaff generator to produce high voltages needed for a few class demonstrations.

I forgot to mention the Kelvin water dropper in class.  This is another apparatus that is able to separate and accumulate charge.  We have a couple of mostly constructed Kelvin water droppers that could be used for a class project.

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We'll concentrate on three of Franklin's contributions:

(1) the power of points

(2) noticing and suggesting a means of verifying that the electricity in thunderstorms and lightning was the same as the electricty produced by the electrostatic machines and being studied in the laboratory, and

(3) his invention of lightning rods.

Priestley

“He (Franklin) was properly the first who observed the entire and wonderful effect of pointed bodies, both in drawing and throwing off the electric fire.”

 

Franklin

“The first is the wonderful effect of pointed bodies, both in drawing off and throwing off the electrical fire.”

This was demonstrated using a Van de Graaff generator

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As soon as a pointed, grounded brass rod is brought to within about 20 centimeters of the Van de Graaff, the sparking to the grounded round ball stops.

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Franklin saw many similarities between the electricity used in his experiments and lightning. 

Both produce light, and the colors of light are similar.  Crooked channels.  Swift motion. Being conducted by metals.  Crack or noise produced during discharge.  "Subsisting" in water or ice. "Rending" bodies as current passes through.  Killing animals.  Melting metals. Catching inflammable materials on fire. Sulphurous smell.

He wondered whether lightning wasn't just a much larger scale form of the same phenomenon and proposed the following experiment (the Sentry box experiment was described in a July 29, 1750 letter)

"To determine the question, whether the clouds that contain lightning are electrified or not, I would propose an experiment to be tried where it may be done conveniently.  On the top of some high tower or steeple, place a kind of sentry-box big enough to contain a man and an electrical stand.  From the middle of the stand let an iron rod rise and pass bending out of the door, and then upright 20 or 30 feet, pointed very sharp at the end.  If the electrical stand be kept clean and dry a man standing on it when such clouds are :passing low, might be electrified and afford sparks, the rod drawing fire to him from a cloud.  If any danger to the man should be apprehended (though I think there would be none) let him stand on the floor of his box, and now and then bring near to the rod the loop of a wire that has one end fastened to the leads, he holding it by a wax handle; so the sparks, if the rod is electrified, will strike from the rod to the wire, and not affect him.”

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figure above is from Uman's 187 book "The Lightning Discharge."

The experiment was performed for the first time on May 10, 1752 in Marly-la-Ville (near Paris) by a retired dragoon name Coiffier (Thomas Francois Dalibard, a naturalist, was absent).  Dalibard read an account of the experiment to the French Academie des Sciences on May 13, 1752. 

 

“En suivant la route que M. Franklin nous a tracé, j’ai obtenu une satisfaction complète.  Voice les préparatifs, le procédé et le succès.

Commune de Marly La Ville website

Google map

The experiment was repeated for the French king, Louis XV, a short time later.

 

The experiment was widely repeated

LeMonnier held a 5 m wooden pole with iron wire windings while standing on pitchcake.  Sparks from hands and face.

Franklin never did the sentry box experiment (he thought the metal rod would need to be higher and came up with the idea of using a kite)

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This figure is also from Uman's 1987 book "The Lightning Discharge"

The experiment is thought to have been conducted in June, 1752, but the exact date and location were never recorded.    Details of the experiment were sent to Collinson in a letter dated Oct. 19, 1752.

Other people began to repeat the experiment using rockets (mortars) and balloons.  In June 1753 de Romas used a kite with a 240 m cord wrapped with violin wire.  He produced 20 cm long sparks.  Apparently later was able to produce 3 m long sparks!

The strength of the electricity was often judged by simulating the muscles of animals and observing their reaction.

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from Ref (1)

Both the sentry box experiment and the kite experiment are very dangerous.  If lightning were to strike the metal pole or the kite or balloon, the person at the bottom would likely be killed.  This did eventually happen

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Figure from ref (2)

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Franklin came up with the idea of lightning rods

“There is something however in the experiments of points, sending off, or drawing on, the electrical fire, which has not been fully explained, and which I intend to supply in my next. For the doctrine of points is very curious, and the effect of them truly wonderful; and, from what I have observed on experiments, I am of opinion, that houses, ships, and even towns and churches may be effectually secured from the stroke of lightning by their means; for if, instead of the round balls of wood or metal, which are commonly placed on the tops of the weather-cocks, vanes or spindles of churches, spires, or masts, there should be put a rod of ion 8 or 10 feet in length, sharpen’d gradually to a point like a needle, and gilt to prevent rusting, or divided  into a number of points, which would be better – the electrical fire, would, I think, be drawn out of a cloud silently, before it could come near enough to strike; only a light would be seen at this point, like the sailors corpusante.” 

 “I say, if these things are so, may not the knowledge of this power of points be of use to mankind, in preserving houses, churches, ship etc. from the stroke of lightning, by directing us to fix on the highest parts of those edifices, upright rods of iron made sharp as a needle, and gilt to prevent rusting, and from the foot of those rods a wire down the outside of the bulding into the ground, or down round one of the shrounds of a ship, and down her side till it reaches the water?”

and my favorite:

"It has pleased God in his goodness to mankind, at length to discover to them the means of securing their habitations and other buildings from mischief by thunder and lightning ..."

Franklin was anticipating opposition from religious authorities because lightning was considered by many to be a form of divine retribution.

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Franklin originally thought (incorrectly) a lightning rod would dissipate electricity (the pointed tip would draw off electricity before a discharge could occur).  The first strike to one of Franklins rods melted the top of the rod which surprised him. 

Metal (nail) rods were often linked together as shown below (some fragments of Franklin's original lightning rods still exist, in one case inside a building and next to dry wooden beams).  The links tended to rupture.

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As problems became apparent Franklin worked to make improvements.  In particular he investigated the following:

how does the rod work?

what material should be used?

termination in air

grounding

attachment to structure

height above the structure

area protected by the rod

Considerable opposition to the use of lightning rods in Europe.  They didn't believe that it would dissipate the electricity (correct).  Franklin argued that even if not, the lightning rod and wire to ground would safely carry the lightning current around and thereby protect the structure.

"Ligtning had been regarded as a divine expression, a manifestation against which there could be no possible protection, except prayer and the ringing of church bells.  Such bells cast in mediaeval times often bore the legend "Fulgura frango" ("I break up the lightning").  With the passage of time, however, it was realized that bell ringing during a storm was a very hazardous remedy, especially for the ringer on the ropes becuase so many were killed by the very stroke they attempted to disperse."  In 33 years of lightning strokes on 386 church steeples 103 bell-ringers were killed. ref (2)

A showdown took place in the Piazza in Siena Italy in Spring 1777.  One side doubted the electrical nature of lightning and the efficacy of lightning rods.  A second, more progressive side, had ordered a lightning rods to be installed on the cathedral and the tower of City Hall (facing the plaza where the famous Palio is run).  "On the afternoon of 18 April clouds began to form, distant thunder was heard, and the Siennese began moving to their Piazza with all eyes focused on the lightning rod tip.  At about five o'clock - lightning struck.  A ball of fire, accompanied by sparks, smoke, and an odor of sulphur ran the full length of the tower and disappeared into the ground leaveing the tower unharmed."

Lightning rods were quickly adopted throughout Italy (and also in other Catholic countries because they were approved by The Pope)

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A lightning house, a common demonstration of the efficacy of lightning rods

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It is hard to appreciate the acclaim that Franklin's ideas and experiments in electricity brought to him in Europe (though he did also have some enemies)

May 1752 Congratulations from the King of France

July 1753 Master of Arts from Harvard University

Sept. 1753 Master of Arts from Yale University

Nov. 1753 Copeley Gold Medal, Royal Society, London

April 1756 Fellow (w/o fee) of the Royal Society

Feb. 1759 University of St. Andrews Scotland Doctor of Civil and Canon Laws

April 1762 Oxford Doctor of Civil Laws

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Note the Franklin chimes (center left) and a grounded lightning rod (window)

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The Franklin chimes apparatus used in class consisted of a center bell surrounded by 4 outer bells.  The four outer bells were all connected together electrically.  The center bell was insulated from the outer bells.

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The small metal ball hung between each of the outer bells and the center bell.  The small balls were assumed initially to be uncharged. 

The top of the Van de Graaff generator was connected to the outer bells.  We assume a positive charge.  This causes charges to be induced on the surface of each of the balls as shown in the figure (these charges make the electric field inside the ball zero).  The balls will be attracted toward the outer bells.

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Once the ball contacts the center bell, the negative charge is neutralized.  The positively charged ball is repelled by the center bell.

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Once the ball touches the grounded bell, the positive charge flows to ground. 

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The ball is again uncharged.  The whole process repeats itself.  The motion of the ball is transporting charge from the center bell (i.e. from the Van de Graaff generator ) to ground.

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The Volta Hailstorm apparatus consisted of two metal disks mounted above each other.  They were mounted inside a clear plastic cylinder.   Small round balls made of aluminum foil were inside.  The apparatus was placed on top of a Van de Graaff generator.   The foil balls are in contact with the top electrode and acquire some charge (assumed to be positive). 

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The balls are repelled by the bottom electrode and travel up to the top metal disk.  They transfer their charge to the top disk and then fall back to the bottom disk.

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The charge on the top disk bleeds off into the air through the pieces of wire connected to the disk (the charge eventually travels to ground and completes the circuit).  The foil balls again acquire charge from the bottom disk and the whole process repeats itself. 

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References

(1) H. Prinz, Lightning in History, Ch. 1 in Lightning Vol. 1, ed. by R.H. Golde, Academic Press, London, 1977.

(2) B. Dibner, Benjamin Franklin, Lightning, Ch. 2 in Lightning Vol. 1, ed. by R.H. Golde, Academic Press, London, 1977.

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